Negative feedback amplifying circuit



NOV. 15,1949 N, WEBSTER 2,488,357

NEGATIVE FEEDBACK AMPLIFYING CIRCUIT Filed May 20, 1947 2 Sheets-Sheet 1 31M) elm tom Nov. 15, 1949 N. D. WEBSTER 2,488,357

NEGATIVE FEEDBACK AMPLIFYING CIRCUIT Filed May 20, 1947 2 Sheets-Sheet 2 "P I I m {M Patented Nov. 15, 1949 UNITED STATES ENT OFFICE NEGATIVE FEEDBACK AMPLIFYING CIRCUIT Application May 20, 1947, Serial No. 749,320

6 Claims.

This invention relates to electronic amplifying systems and particularly to improved circuits for the same.

One object of the invention is to provide a circuit in which a single-ended amplifier may be utilized to produce a large amount of power at audio frequencies and at low distortion levels, while at the same time providing a low impedance source of audio frequency signal energy.

Another object of the invention is to provide a circuit of the above type in which a beam pentode cathode follower tube is employed to drive a single-ended class A beam pentode amplifier. By connecting the screen grid of the cathode follower through a voltage dropping resistor to the anode of an output amplifier the linearity of the amplifier tube may be increased greatly over the values obtainable by prior art circuits where single-ended amplifiers employing pentodes are utilized.

Other objects include the proportioning of parts so that large amounts of negative feedback may be used while maintaining low distortion levels, the use of circuits to prevent peak clipping in the cathode follower and various other features which make the circuit useful in relations where known circuits are unsatisfactory. These objects and advantages will be more fully understood from the following description when it is read in conjunction with the accompanying drawings in which,

v Figure 1 is a circuit diagram showing a complete amplifying circuit embodying the present invention.

Figure 2 is a diagram showing the basic elements only of the improved amplifier of Figure l.

The circuit about to be described is intended primarily for use in audio frequency amplification of speech or music where faithful reproduction and freedom from distortion are of paramount importance. Although the circuit was evolved for the use indicated, no limitation to that use is intended. The description which follows is, therefore, to be considered as exemplary only and without limitations except where they are required by the prior art and by the language of the claims when construed in the light of that art.

In describing the invention reference will first be had to the circuit of Figure 2 where reference characters 3 and 4 designate the input terminals for supplying signal energy, from a suitable source, over resistor 5 to a circuit comprising amplifier pentode A, cathode follower tube B and amplifier C. As here shown the tubes 13 and C are of the beam pentode type and they are preferred because of their efficiency, but the use of pentodes utilizing a suppressor grid is contemplated where such use is feasible. The output of the last amplifier tube, in this case tube C, delivers amplified signal energy to terminals 44 and i5 which are connected to the outgoing line or circuit. The cathode heater circuits are omitted for purposes of simplification, and like reference characters indicate like parts in the two views.

The amplifier pentode A has a control grid 6, cathode 7, screen grid 8, suppressor grid 9 and anode ill. Anode current for the tube A, as well as for the tubes B and C, is supplied from the 13-!- source. In connection with tube A the anode current from the B+ source passes through line H, inductance l2, tube A, and resistor I3 to ground. Grid bias for tube A is provided through resistors 5 and i3. Screen bias for tube A is provided through parasitic suppressor resistor 4 and dropping resistor 55, with bypass condenser l6 connected to ground at ll.

Cathode follower tube B, of the beam power type having an anode 20, a screen grid 2|, a control grid 22 and a cathode 23, is supplied with anode current from the 13-]- source, over line H, resistor 24 and inductance 25, and its grid bias is supplied by voltage drop across resistor 24, and the resistive portion of inductance 25 through resistor l8. Its screen bias is supplied from line 21 through resistor 26. I9 is a coupling condenser between the output of tube A and the input of tube B.

The tube C which here serves as the last amplifier tube and is representative of a single-ended amplifier which may comprise several parallelconnected tubes as shown in Figure 1, has an anode 33, a screen grid 34!, a control grid 35 and a cathode 31. It receives its anode current from the B+ source through inductance 4|, resistor 3|, and resistor 38. Grid bias for tube C is provided by the voltage drop across resistor 38, through resistor 36 and inductance 25, the resistor 38 being shunted by condenser. 39. Biasing potential for the screen grid 34 of tube C is supplied through resistor 40.

As indicated above this circuit provides negative feedback which is peculiarly effective at high values without raising the output distortion level. This feedback connection includes line 21, resistor 28 and stoppin condenser 29, connecting the anode 33 of tube C with the cathode of tube and the end of resistor I3 at point 30. This ly manifest when a pentode is operated at in-.

put levels that approach anode current cut-off during negativehalf cycles of input signal.

In order to compensate for this characteristic distortion, control is effected by resistor 26 from the anode 33 of tube C to the screen grid. 2| of tube B. This feedback to the screen grid of tube B controls the transconductance of the tube 13. Since tube B is a cathode follower its transconductance and gain can be lowered to a considerable extent by application of negative voltage to the screen grid, but positive voltage applied to the screen cannot raise the gain of the tube appreciably since the gain of a cathode follower is limited to a value less than unity. Positive excursions of voltage on the screen tube B do, however, prevent negative peak clipping in this tube. Consequently tube B feeds a distorted signal to the tube C, but this distortion is such as to compensate for the character distortion which is inherent in the tube C, a d as a result, the output from tube C is relatively free from distortion,

For example, during positive grid voltage excursions in tube C, a negative voltage is fed from the anode of tube C to the screen grid of tube B. This reduces the transconductance of tube B which in turn cuts down the voltage across inductance 25 limiting the positive grid voltage swing of tube C. When the control grid 35 of tube C goes negative a positive voltage is fed to the screen 2| of tube B. This increases the transconductance of tube B, but since the gain of tube 13 can never exceed unity. positive changes in the screen grid of tube .8 have but a slight effect on the voltage fed to the control grid of tube C. This ositive voltage does, however, prevent negative peak clipping which may otherwise occur in the cathode follower. By proper choice of tubes and circuit values the output of tube C may be made very large, while overall grid voltage-anode current linearity may be retained for an extended range of anode load impedances. It is possible with this arrangement to feed reactive loads such as high fidelity loud speaker systems employing crossover networks, or transcription cutting heads without appreciably increasing the R. M. S. harmonic or intermodulation distortion of the amplifier.

This amplifier, because it operates differently from the conventional single-ended amplifier employing negative feedback, is able to develop relatively large amounts of power at low distortion levels. To obtain large power outputs from conventional singleeended amplifiers employing pentodes, it is necessary that the grid of the output tube be driven toward positive sufficiently to cause the anode voltage to drop to a minimum level where further increases in positive grid voltage have little reaction on anode current. This condition results in the negative half cycle of the output. wave possessing a fiat-topped characteristic. It is intended that negative feedback shall corre th s condition; however, a further posi- 4 tive grid excursion will cause no additional drop in the instantaneous anode voltage. Hence, such negative feedback tends to make the wave shape more fiat-topped than ever. Thus as ordinarily used, the negative feedback will, as the output approaches maximum power levels, increase rather than decrease distortion. The design of this improved amplifier is such that positive signals on the grid of tube C are limited by the controlling action of tubes 13 and C as previously described. Therefore, tube C may operate to anode current cutoff on negative grid excursions, and to the point where the Eg-Ip relation becomes non-linear during positive grid excursions. Hence, this amplifier makes it possible to utilize the full useful Eg-Ip characteristic without peak clipping, and as a result the negative feedback is effective in reducing distortion components. This amplifier is in optimum adjustment when (at an output power beyond maximum undistorted power) negative and positive peak clipping of the output waveform occur at the same value of sinusoidal voltage input to the amplifier.

The amplifier system has the advantage of being able to feed large amounts of negative feedback from the anode of tube C through resistor 28 and condenser 29 to the cathode of tube A without encountering a fiat-top output wave condition or the usual problem of transient distortion. This is possible because the amplifier is arranged to utilize the full useful portion of the characteristic of tube C, and at the same time, has a far greater frequency response than amplifiers employing push-pull arrangements wherein the desired effects of negative feedback are partly nullified by phase shift at high audio frequencies, the shift being caused by incomplete coupling and consequent leakage reactance between the two primary halves of the output transformer. The large amount of negative feedback that is possible with this amplifier is useful in driving loudspeaker systems, since any counter E. M. F. distortion developed by the loudspeaker is largely corrected by the amplie fier, and as a result, loudspeaker cone overshoot is reduced by a factor proportional to the conversion eificiency of the speaker.

While the circuit shown in Figure 2 is a complete operative unit, and embodies the principal features of the invention the modification of Figure 1 incorporates certain minor changes and additions which will be found useful in practice. Inasmuch as the overall circuit of Figure l incorporates all of the structure of Figure 2, the present description will be restricted to the additions.

In Figure 1 the tubes A, B, and C correspond exactly to the tubes similarly designated in F gure 2. However, the control grid biasing circuit of tube B embodies two inductances 46 and 41, instead of the single inductance !2 of Figure 2. These two inductances are arranged to buck each other for the purpose of balancing out the hum which might otherwise be objectionable.

The audio amplifier unit. C of Figure 2 comprises four tubes, C, Cl, C2 and C3, all of like construction and connected in parallel, so as to comprise a single-ended amplifier made up of a plurality of tubes but possessing he charac ristics already described. The filter circuit comprising condenser 4 and resist r it in he utput of tube C in Figure 2, is modified by the addition of, a Second resistor 48, the condenser being connected between the two resistors and grounded as before.

The input to tube A is supplied from a suitable transformer 49, the secondary of which feeds the control grid of the first audio stage AF, here shown as a tetrode in which two grids are connected to the anode to give a low mu value. bviously other conventional tubes could be substituted. The anode of tube AF supplies the grid of tube A and the anode circuit includes a filter network comprising a grounded condenser 50 connected between series resistors 51 and 52.

A conventional source of B supply is indicated by transformers 53 and 54 and rectifier tubes 55, 56, 51, and 58, including suitable filters represented by inductances 59, and 60, and condensers GI and 62. The cathode circuit of each tube, for convenience, contains a test jack 63 for use in ascertaining the condition of any tube in the circuit.

The operation of the circuit shown in Figure 1, so far as the present invention is concerned, corresponds precisely to that which has been described in connection with Figure 2, hence repetition is believed to be unnecessary.

The invention has been described as provided by the patent statutes, but it is to be understood that changes may be made in the details, within the scope of the appended claims, without departing from the spirit and scope of the invention.

Having thus described my invention, I claim:

1. An amplifier circuit comprising an output pentode amplifier tube having input and output circuits, a load in said output circuit, a pentode cathode follower tube having its screen grid connected through a voltage dropping resistance capable of passing the alternating current component of the output of said pentode amplifier tube to the anode of the amplifier tube, a cathode follower connection from the cathode circuit of the cathode follower tube to the grid circuit of the amplifier tube, an input amplifier pentode for supplying energy to the input of the cathode follower, and a negative feedback connection from said anode of said amplifier tube to the cathode of said input tube, said feedback connection including a condenser and a resistor in series.

2. An amplifier circuit comprising a source of electric wave energy, an input tube having its input connected to said source, a pentode cathode follower tube connected to the output of said input tube, a pentode amplifier tube having a load in its output, resistance means connecting the screen grid of the cathode follower to the anode of the amplifier tube and capable of passing the alternating current component of the output of said pentode amplifier tube, a cathode follower connection from the cathode circuit of the oathode follower to the grid circuit of the amplifier tube, and a negative feedback connection from the output of the amplifier tube to the cathode of the input tube, said connection including a condenser and a resistor in series.

3. An amplifier circuit comprising an audio frequency amplifier having input and output circuits, a source of electric wave energy connected to said input, a pentode cathode follower having its input connected to the output of said audio amplifier, a class A pentode amplifier having its input connected to the cathode circuit of the cathode follower, a load connected to the output of said pentode amplifier, a negative feedback connection from the output of the pentode amplifier to the input of the audio frequency amplifier, and means including a voltage dropping resistor capable of passing the alternating current component of the output of said pentode amplifier for connecting the screen grid of the cathode follower to the anode of the pentode amplifier.

4. An amplifier circuit as set forth in claim 3 where the class A pentode amplifier is made up of a plurality of parallel connected tubes.

5. An amplifier circuit comprising a singleended class A pentode amplifier having a load connected to its output, a pentode cathode follower tube connected to drive said amplifier, the driving connection including a circuit between the screen grid of the cathode follower and the anode of the amplifier and including a voltage dropping resistor capable of passing the alternating current component of the output of the pentode amplifier, a source of energy, an input amplifier supplied by said source, and a negative feedback connection from the output of the pentode amplifier to the input of said input amplifier.

6. An amplifying circuit comprising a cathode follower tube having a screen grid and input and output circuits, and an output amplifier, the output of said cathode follower being connected to the control grid of said output amplifier and the anode of the output amplifier being connected to the screen grid of said cathode follower tube through a voltage dropping resistor capable of passing the alternating current component of the output of said output amplifier, whereby a portion of such alternating current component is fed back to the screen grid of the cathode follower tube.

NORMAN D. WEBSTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,198,323 Wagner Apr. 23, 1940 2,198,464 Shepard, Jr Apr. 23, 1940 2,210,390 Weathers Aug. 6, 1940 2,232,850 Haantjes et a1 Feb. 25, 1941 

